14G Mr. F. Merrifield's systematic temperature 



conclusions appear to be well-founded, and I hope that 

 so many recorded facts, the substantial accuracy of 

 which may be relied on, may prove useful in other 

 ways to some of those who are prosecuting biological 

 researches. 



I. As to seasonal douhle-bwodcd species. — This term is 

 used as a name, not a definition, and in the restricted 

 sense of a species which has two (or more) generations 

 in a year, one of them passing the pupal stage in the 

 winter, the insects belonging to this generation, which I 

 will call the winter type, differing from those belonging 

 to the summer generation, which I will call the sum- 

 mer type, not only in (1) the season of the year 

 during which their existence is passed, and (2) the 

 duration of that existence, sometimes three or four 

 times as long as that of the summer type, but also often 

 presenting differences in (3) size, (4) shape, (5) colour, 

 and (6) markings. (The sense in which I use the term 

 excludes certain species of which I have little knowledge, 

 such, for exami)le, as those which are many-brooded in 

 countries where there is no real winter ; also those 

 species which pass the winter in some other than the 

 pupal stage.) 



1. In such a species a young individual may have, 

 and often has, a constitutional capacity for developing 

 into either type, according to external circumstances. 



2. It seems probable that there is from the beginning 

 of the existence of the individual a tendency, which 

 may be very strong, or very slight, to develop in the 

 direction of one of the two types. 



3. If there is no such innate tendency in an indi- 

 vidual, it can be imparted by external influences during 

 the early part of its existence. 



4. "Where the tendency exists, it varies in strength in 

 different individuals. In the case of some species, or 

 some broods or individuals of some species, the tendency 

 from the beginning is so strong that it cannot be over- 

 come by any external influences. 



5. In other cases the tendency can be overcome and 

 converted into the opposite one, or turned more or less 

 in the direction of it, by such influences. The decision 

 as to the type to be assumed is come to before the 

 termination of the growth of the larva, and this decision 

 may be completely controlled in some cases by external 

 influences applied before that period. For example, in 



